Some 800 people are believed to have died in the worst monsoon floods in living memory.
Officials say 19,000 people in the worst-hit areas have been rescued by 30,000 troops.
But there are now concerns that if forecast heavy rain arrives it will push the flood south to Sindh province.
Currently Peshawar, the largest city in the north-western region, with a 3m-strong population, is cut off.
Officials estimate that water levels in the southern province of Sindh could reach "exceptionally high flood" in four or five days as water draining down from the north-west is compounded by more rain.
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In pictures: Pakistan floods
The head of Pakistan's army engineers said that while water should recede in the north, a new system of monsoon weather was due from Sunday.
A "major deluge" would pass through Sukkur, in Sindh, from Tuesday to Thursday, it was reported.
As well as hundreds of deaths in Pakistan, at least 60 people have died across the border in Afghanistan, where floods affected four provinces.
Washed away
On Saturday, Mian Iftikhar Hussain, information minister for Pakistan's Khyber-Pakhtoonkhwa (formerly North-West Frontier) province, announced the latest death toll, and described he floods as the province's worst ever.
Manuel Bessler, the head of the UN's Office for the Coordination for Humanitarian Assistance (UNOCHA) in Pakistan, told the BBC about 1m people's lives had been disrupted.
He could not say with certainty the full scale of the emergency in Pakistan, as he was having trouble reaching his own offices in some of the worst-affected areas.
UN aid workers were helping to co-ordinate efforts to provide shelter, health care, drinking water and ready-to-eat food rations, he said.
The government declared a state of emergency as Pakistan's meteorological department said 312mm (12in) of rain had fallen over 36 hours in the north-west - the largest amount for decades.
The districts of Swat and Shangla have been inaccessible with people left homeless and helpless after several rivers burst their banks, washing away villages, roads and bridges. Some 45 bridges were washed away in Swat alone.
The BBC's Lyse Doucet, who is travelling through some of the worst-hit areas, says at least half a million people remain marooned on islands of high ground, while others have taken refuge in mosques and schools.
TV footage taken from helicopters flying over the flooded landscape showed people clinging to roof-tops as raging torrents swept through the streets.
Military and rescue workers have been using helicopters to deliver essential supplies to areas that have had transport and communication links cut off.
Some 17 helicopters were in action to airlift people out of the worst affected areas on Friday and more were being deployed over the weekend.
Swathes of farmland have been inundated, and some power supplies have been cut after people were electrocuted by the water-borne current.
Many of those hit hardest by the flooding are the rural poor who live in flood-prone areas because they cannot afford safer land.
Pakistan has not made a formal request for international aid, but it is understood that it has appealed to donors to help it respond to this disaster.
Afghan effort
In Afghanistan, the national army said it had rescued 5,000 people over the past three days, using helicopters, vehicles and bulldozers.
The provinces of Laghman, Nangarhar, Kunar and Logar have all been hit by the bad weather.
There were plans to deliver food and medicine on Monday but the mountainous terrain was hindering the effort.
In Eastern Logar province, a provincial spokesman told the BBC that 10 people had been killed overnight. Nomad communities had lost tents and livestock, he added.
In Kama, Nangarhar, local resident Haji Baqi told the BBC: ''We lost all of our food.
"I lost three wheat harvests, our bridges have been destroyed. We want the government to come and help. What will people eat for the rest of the year? Where is the government? When are they going to help us.''
